Ultraviolet (UV) illumination (254 nm) induced production of the phytoalexin scoparone in flavedo of kumquat (Fortunella margarita Lour. Swingle cv. Nagami) and orange [Citrus sinensis (L.) Osbeck cvs. Shamouti and Valencia]. Trace amounts of scoparone (<2.0 μg·g-1 fresh weight of flavedo) were detected in nontreated fruits. Phytoalexin accumulation in kumquat reached a peak of 530 μg·g-1 11 days after illumination, hut the amount declined rapidly, returning to a trace level 1 month after treatment.. Production of scoparone in illuminated fruits was enhanced by increasing the UV dose from 1.5 × 103 to 9.0 × 103 J·m-2 for orange and from 0.2 × 103 to 1.5 × 103 J·m-2 for kumquat and by raising the storage temperature from 2 to 17C. Phytoalexin accumulation correlated with an increase in antifungal activity of flavedo extracts. UV-illuminated kumquat fruit inoculated with Penicillium digitatum Sacc. 2 days after treatment had a lower incidence of decay than the control. Illumination of previously inoculated fruit failed to prevent decay. Kumquat fruit stored at 17C showed signs of UV-induced peel damage. Chemical name used: 6,7-dimethoxycoumarin (scoparone).
Victor Rodov, Shimshon Ben-Yehoshua, Jong Jin Kim, Boris Shapiro, and Yitzhak Ittah
Samir Droby, Ron Porat, Lea Cohen, Batia Weiss, Boris Shapiro, Sonia Philosoph-Hadas, and Shimon Meir
Jasmonic acid (JA) and methyl jasmonate (MJ), collectively referred to as jasmonates, are naturally occurring plant growth regulators involved in various aspects of plant development and responses to biotic and abiotic stresses. In this study, we found that postharvest application of jasmonates reduced decay caused by the green mold Penicillium digitatum (Pers.: Fr.) Sacc. after either natural or artificial inoculation of grapefruit (Citrus paradisi `Marsh Seedless'). These treatments also effectively reduced chilling injury incidence after cold storage. The most effective concentration of jasmonates for reducing decay in cold-stored fruit or after artificial inoculation of wounded fruit at 24 °C was 10 μmol·L-1. Higher and lower jasmonate concentrations were less effective at both temperatures. MJ at 10 μmol·L-1 also most effectively reduced the percentage of fruit displaying chilling injury symptoms after 6 weeks of storage at 2 °C and 4 additional d at 20 °C. When tested in vitro, neither JA nor MJ had any direct antifungal effect on P. digitatum spore germination or germ tube elongation. Therefore, it is suggested that jasmonates probably reduced green mold decay in grapefruit indirectly by enhancing the natural resistance of the fruit to P. digitatum at high and low temperatures.